Method of continuously casting thin strip

ABSTRACT

A method of continuously casting steel including steps of forming a casting pool of molten steel comprising a carbon content of less than 0.5% by weight on casting surfaces of a pair of internally cooled casting rolls having a nip formed between them, counter rotating the casting surfaces of the casting rolls toward each other to produce a cast steel strip moving downwardly away from the nip between the casting rolls, guiding the cast strip through a first enclosure adjacent the casting rolls as the strip moves away from the casting rolls, the first enclosure having a reducing atmosphere containing carbon monoxide and optionally hydrogen of at least 0.1%, establishing the reducing atmosphere in the first enclosure to control ingress of atmospheric air so as to maintain said atmosphere with a CO to CO 2  ratio of at least 1.5 during steady state operation.

This application is a divisional application of U.S. patent application Ser. No. 13/040,863 filed on Mar. 4, 2011, which is incorporated herein by reference.

BACKGROUND AND SUMMARY

This invention relates to the casting of metal strip by continuous casting in a twin roll caster.

In a twin roll caster molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.

When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures of the order of 1400° C. and can suffer very rapid scaling due to oxidation at such high temperatures in an air atmosphere. Such scaling may result in a significant loss of steel product. Moreover, such scaling results in the need to descale the strip prior to further processing by pickling to avoid surface quality problems such as rolled-in scale, and causes significant extra complexity, cost and environmental concerns.

To deal with the problem of rapid scaling of strip emerging from a twin roll strip caster, it has been proposed to enclose the newly formed strip within a sealed enclosure, or a succession of such sealed enclosures, in which a controlled atmosphere or atmospheres is maintained in order to inhibit oxidation of the cast strip. The controlled atmosphere can be produced by charging the sealed enclosure or successive enclosures with non-oxidizing gases.

Such sealed enclosures, however, still allow ingress of some outside air, causing oxidation that increases heat loss from the strip. There remains a need for further control of the atmosphere in the enclosures downstream of the nip.

Disclosed is a method of continuously casting steel comprising:

-   -   (a) forming a casting pool of molten steel comprising a carbon         content of less than 0.5% by weight on casting surfaces of a         pair of internally cooled casting rolls having a nip formed         between them,     -   (b) counter rotating the casting surfaces of the casting rolls         toward each other to produce a cast steel strip moving         downwardly away from the nip between the casting rolls,     -   (c) guiding the cast strip through a first enclosure adjacent         the casting rolls as the strip moves away from the casting         rolls, the first enclosure having a reducing atmosphere         containing carbon monoxide of at least 0.1% and optionally         hydrogen of at least 0.1%,     -   (d) establishing said reducing atmosphere in the first enclosure         during steady state operation to control ingress of atmospheric         air so as to maintain said atmosphere with a CO to CO₂ ratio of         at least 1.5 during.

The reducing atmosphere having and oxygen level of less than 0.5% in the first enclosure may also contain argon with an O₂ to Ar ratio of less than 18 during steady state operation. Alternatively, the reducing atmosphere in the first enclosure may have an O₂ to Ar ratio between 10 and 15 during steady state operation. Alternatively, the reducing atmosphere in the first enclosure may have an oxygen level of less than 0.25%.

The atmosphere in the first enclosure may have a CO to CO₂ ratio of at least 2.5 during steady state operation.

In one alternative, the molten steel comprises a carbon content of less than 0.1% by weight.

Alternatively, a method of continuously casting steel may comprise:

-   -   (a) forming a casting pool of silicon killed molten steel on         casting surfaces of a pair of internally cooled casting rolls         having a nip formed between them,     -   (b) counter rotating the casting surfaces of the casting rolls         toward each other to produce a cast steel strip moving         downwardly away from the nip of the casting rolls such that iron         silicate is formed on the surface of the cast strip,     -   (c) guiding the cast strip through a first enclosure adjacent         the casting rolls as the strip moves away from the casting         rolls, the first enclosure having a reducing atmosphere         containing carbon monoxide of at least 0.1% and optionally         hydrogen of at least 0.1%, and     -   (d) establishing said reducing atmosphere in the first enclosure         during steady operation to control ingress of atmospheric air so         as to maintain said atmosphere with a CO to CO₂ ratio of at         least 1.5.

The reducing atmosphere in the first enclosure may also contain argon with an average O₂ to Ar ratio of less than 18 during steady state operation. Alternatively, The atmosphere in the first enclosure may have an average O₂ to Ar ratio between 10 and 15 during steady state operation.

The reducing atmosphere in the first enclosure may also contain an average CO to CO₂ ratio of at least 2.5 during steady state operation.

The molten steel may have a carbon content of less than 0.5% by weight. Alternatively, the molten steel has a carbon content of less than 0.5% by weight.

Also disclosed is a method of continuously casting steel comprising:

-   -   (a) forming a casting pool of molten steel comprising iron and         silicon on casting surfaces of a pair of internally cooled         casting rolls having a nip formed between them,     -   (b) counter rotating the casting surfaces of the casting rolls         toward each other to produce a cast steel strip moving         downwardly away from the nip between the casting rolls such that         iron silicate is formed on the casted surface of the cast strip,     -   (c) guiding the cast strip through a first enclosure adjacent         the casting rolls as the strip moves away from the casting         rolls, the first enclosure having a reducing atmosphere         containing carbon monoxide of at least 0.1% and optionally         hydrogen of at least 0.1% to control ingress of atmospheric air         so the atmosphere in the first enclosure has a CO to CO₂ ratio         of at least 1.5 during steady state operation,     -   (d) moving the cast strip through pinch rolls and thereafter         through a second enclosure upstream of a roll mill where the         cast strip reduction is at least 10%, the atmosphere in the         second enclosure being a controlled atmosphere containing a         total of oxygen, water vapor and hydrogen of greater than 8% by         volume during steady state operation.

The method may further include the step of moving the cast strip through an intermediate enclosure between the first enclosure and the second enclosure, the intermediate enclosure being a reducing atmosphere containing carbon monoxide and/or hydrogen of at least 0.1%.

A measured temperature in the first enclosure adjacent the pinch rolls may be between about 1800 and 2400° F.

The reducing atmosphere in the first enclosure may also contain argon with an O₂ to Ar ratio of less than 18 during steady state operation. Alternatively, the reducing atmosphere in the first enclosure may have an O₂ to Ar ratio between 10 and 15 during steady state operation.

The atmosphere in the first enclosure may also contain argon with an average CO to CO₂ ratio of at least 2.5 during steady state operation.

The molten steel comprises a carbon content of less than 0.5% by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatical side view of a twin roll caster of the present disclosure,

FIG. 2 is a diagrammatical plan view of the twin roll caster of FIG. 1,

FIG. 3 is a partial sectional view through a pair of casting rolls mounted in a roll cassette of the present disclosure,

FIG. 4 is a diagrammatical side view of the first enclosure of the caster beneath the casting rolls,

FIG. 5 is a diagrammatical side view of the second and intermediate enclosures between a pinch roll and hot rolling mill,

FIG. 6 is a graph of formation of iron oxides as a function of CO in a CO+CO₂ mixture and temperature, and

FIG. 7 is a graph of formation of iron oxides on silicon killed steel as a function of CO in a CO+CO₂ mixture and temperature.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGS. 1 through 7, a twin roll caster is illustrated that comprises a main machine frame 10 that stands up from the factory floor and supports a pair of casting rolls mounted in a module in a roll cassette 11. The casting rolls 12 are mounted in the roll cassette 11 for ease of operation and movement as described below. The roll cassette facilitates rapid movement of the casting rolls ready for casting from a setup position into an operative casting position in the caster as a unit, and ready removal of the casting rolls from the casting position when the casting rolls are to be replaced. There is no particular configuration of the roll cassette that is desired, so long as it performs that function of facilitating movement and positioning of the casting rolls as described herein.

As shown in FIG. 3, the casting apparatus for continuously casting thin steel strip includes a pair of counter-rotatable casting rolls 12 having casting surfaces 12A laterally positioned to form a nip 18 there between. Molten metal is supplied from a ladle 13 through a metal delivery system to a metal delivery nozzle 17, or core nozzle, positioned between the casting rolls 12 above the nip 18. Molten metal thus delivered forms a casting pool 19 of molten metal above the nip supported on the casting surfaces 12A of the casting rolls 12. This casting pool 19 is confined in the casting area at the ends of the casting rolls 12 by a pair of side closures or side dam plates 20 (shown in dotted line in FIG. 3). The upper surface of the casting pool 19 (generally referred to as the “meniscus” level) may rise above the lower end of the delivery nozzle 17 so that the lower end of the delivery nozzle is immersed within the casting pool. The casting area includes the addition of a protective atmosphere above the casting pool 19 to inhibit oxidation of the molten metal in the casting area.

The delivery nozzle 17 is made of a refractory material such as alumina graphite. The delivery nozzle 17 may have a series flow passages adapted to produce a suitably low velocity discharge of molten metal along the rolls and to deliver the molten metal into the casting pool 19 without direct impingement on the roll surfaces. The side dam plates 20 are made of a strong refractory material and shaped to engage the ends of the rolls to form end closures for the molten pool of metal. The side dam plates 20 may be moveable by actuation of hydraulic cylinders or other actuators (not shown) to bring the side dams into engagement with the ends of the casting rolls.

Referring now to FIGS. 1 and 2, the ladle 13 typically is of a conventional construction supported on a rotating turret 40. For metal delivery, the ladle 13 is positioned over a movable tundish 14 in the casting position to fill the tundish with molten metal. The movable tundish 14 may be positioned on a tundish car 66 capable of transferring the tundish from a heating station 69, where the tundish is heated to near a casting temperature, to the casting position. A tundish guide positioned beneath the tundish car 66 to enable moving the movable tundish 14 from the heating station 69 to the casting position.

The tundish car 66 may include a frame adapted to raising and lowering the tundish 14 on the tundish car 66. The tundish car 66 may move between the casting position to a heating station at an elevation above the casting rolls 12 mounted in roll cassette 11, and at least a portion of the tundish guide may be overhead from the elevation of the casting rolls 12 mounted on roll cassette 11 for movement of the tundish between the heating station and the casting position.

The movable tundish 14 may be fitted with a slide gate 25, actuable by a servo mechanism, to allow molten metal to flow from the tundish 14 through the slide gate 25, and then through a refractory outlet shroud 15 to a transition piece or distributor 16 in the casting position. The distributor 16 is made of a refractory material such as, for example, magnesium oxide (MgO). From the distributor 16, the molten metal flows to the delivery nozzle 17 positioned between the casting rolls 12 above the nip 18.

The casting rolls 12 are internally water cooled so that as the casting rolls 12 are counter-rotated, shells solidify on the casting surfaces 12A as the casting surfaces move into contact with and through the casting pool 19 with each revolution of the casting rolls 12. The shells are brought together at the nip 18 between the casting rolls to produce a solidified thin cast strip product 21 delivered downwardly from the nip. FIG. 1 shows the twin roll caster producing the thin cast strip 21, which passes across a guide table 30 to a pinch roll stand 31, comprising pinch rolls 31A. Upon exiting the pinch roll stand 31, the thin cast strip may pass through a hot rolling mill 32, comprising a pair of reduction rolls 32A and backing rolls 32B, where the cast strip is hot rolled to reduce the strip to a desired thickness, improve the strip surface, and improve the strip flatness. The rolled strip then passes onto a run-out table 33, where it may be cooled by contact with water supplied via water jets or other suitable means, not shown, and by convection and radiation. In any event, the rolled strip may then pass through a second pinch roll stand (not shown) to provide tension of the strip, and then to a coiler.

At the start of the casting operation, a short length of imperfect strip is typically produced as casting conditions stabilize. After continuous casting is established, the casting rolls are moved apart slightly and then brought together again to cause this leading end of the strip to break away forming a clean head end of the following cast strip. The imperfect material drops into a scrap receptacle 26, which is movable on a scrap receptacle guide. The scrap receptacle 26 is located in a scrap receiving position beneath the caster and forms part of a sealed first enclosure 27 as described below. The first enclosure 27 is typically water cooled. At this time, a water-cooled apron 28 that normally hangs downwardly from a pivot 29 to one side in the first enclosure 27 is swung into position to guide the clean end of the cast strip 21 onto the guide table 30 that feeds it to the pinch roll stand 31. The apron 28 is then retracted back to its hanging position to allow the cast strip 21 to hang in a loop beneath the casting rolls in the first enclosure 27 before it passes to the guide table 30 where it engages a succession of guide rollers.

The sealed first enclosure 27 is formed by a number of separate wall sections that fit together at various seal connections to form a continuous enclosure wall that permits control of the atmosphere within the enclosure. Additionally, the scrap receptacle 26 may be capable of attaching with the first enclosure 27 so that the enclosure is capable of supporting a protective atmosphere immediately beneath the casting rolls 12 in the casting position. The first enclosure 27 includes an opening in the lower portion of the enclosure, lower enclosure portion 44, providing an outlet for scrap to pass from the enclosure 27 into the scrap receptacle 26 in the scrap receiving position. The lower enclosure portion 44 may extend downwardly as a part of the first enclosure 27, the opening being positioned above the scrap receptacle 26 in the scrap receiving position.

A rim portion 45 may surround the opening of the lower enclosure portion 44 and may be movably positioned above the scrap receptacle, capable of sealingly engaging and/or attaching to the scrap receptacle 26 in the scrap receiving position. The rim portion 45 is in selective engagement with the upper edges of the scrap receptacle 26, which is illustratively in a rectangular form, so that the scrap receptacle may be in sealing engagement with the first enclosure 27 and movable away from or otherwise disengageable from the scrap receptacle as desired.

A lower plate 46 may be operatively positioned within or adjacent the lower enclosure portion 44 to permit further control of the atmosphere within the enclosure when the scrap receptacle 26 is moved from the scrap receiving position and provide an opportunity to continue casting while the scrap receptacle is being changed for another. The lower plate 46 may be operatively positioned within the first enclosure 27 adapted to closing the opening of the lower portion of the enclosure, or lower enclosure portion 44, when the rim portion 45 is disengaged from the scrap receptacle. Then, the lower plate 46 may be retracted when the rim portion 45 sealingly engages the scrap receptacle to enable scrap material to pass downwardly through the first enclosure 27 into the scrap receptacle 26. The lower plate 46 may be in two plate portions as shown in FIGS. 1 and 4, pivotably mounted to move between a retracted position and a closed position, or may be one plate portion as desired. A plurality of actuators (not shown) such as servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms and rotating actuators may be suitably positioned outside of the first enclosure 27 adapted to moving the lower plate in whatever configuration between a closed position and a retracted position. When sealed, the first enclosure 27 and scrap receptacle 26 are filled with a desired gas, such as nitrogen, to reduce the amount of oxygen in the enclosure and provide a protective atmosphere for the cast strip.

The first enclosure 27 may include an upper collar portion 43 supporting a protective atmosphere immediately beneath the casting rolls in the casting position. The upper collar portion 43 may be moved between an extended position adapted to supporting the protective atmosphere immediately beneath the casting rolls and an open position enabling an upper cover 42 to cover the upper portion of the enclosure 27. When the roll cassette 11 is in the casting position, the upper collar portion 43 is moved to the extended position closing the space between a housing portion 53 adjacent the casting rolls 12, as shown in FIG. 3, and the first enclosure 27 by one or a plurality of actuators (not shown) such as servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, and rotating actuators. The upper collar portion 43 may be water cooled.

The upper cover 42 may be operably positioned within or adjacent the upper portion of the first enclosure 27 capable of moving between a closed position covering the enclosure and a retracted position enabling cast strip to be cast downwardly from the nip into the first enclosure 27 by one or more actuators 59, such as servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, and rotating actuators. When the upper cover 42 is in the closed position, the roll cassette 11 may be moved from the casting position without significant loss of the protective atmosphere in the enclosure. This enables a rapid exchange of casting rolls, with the roll cassette, since closing the upper cover 42 enables the protective atmosphere in the enclosure to be preserved so that it does not have to be replaced.

The casting rolls 12 mounted in roll cassette 11 are capable of being transferred from a set up station 47 to a casting position through a transfer station 48, as shown in FIG. 2. The casting rolls 12 may be assembled into the roll cassette 11 and then moved to the set up station 47, where at the set up station the casting rolls mounted in the roll cassette may be prepared for casting. At the transfer station 48, casting rolls mounted in roll cassettes may be exchanged, and in the casting position the casting rolls mounted in the roll cassette are operational in the caster. A casting roll guide is adapted to enable the transfer of the casting rolls mounted in the roll cassette between the set up station and the transfer station, and between the transfer station and the casting position. The casting roll guides may comprise rails on which the casting rolls 12 mounted in the roll cassette 11 are capable of being moved between the set up station and the casting position through the transfer station. Rails 55 may extend between the set up station 47 to the transfer station 48, which may include a turntable 58, and rails 56 may extend between the transfer station 48 to the casting position. The casting rolls mounted in a roll cassette may be raised or lowered into the casting position. In one embodiment, the roll cassette 11 may include wheels 54 capable of supporting and moving the roll cassette on the rails 55, 56.

The roll cassette 11 comprises a cassette frame 52, roll chocks 49 capable of supporting the casting rolls 12 and moving the casting rolls on the cassette frame, and the housing portion 53 positioned beneath the casting rolls capable of supporting a protective atmosphere in the first enclosure 27 immediately beneath the casting rolls during casting. The housing portion 53 is positioned corresponding to and sealingly engaging an upper portion of the first enclosure 27 for enclosing the cast strip below the nip.

A roll chock positioning system is provided on the main machine frame 10 having two pairs of positioning assemblies 50 that can be rapidly connected to the roll cassette adapted to enable movement of the casting rolls on the cassette frame 52, and provide forces resisting separation of the casting rolls during casting. The positioning assemblies 50 move at least one of the casting rolls 12 toward or away from the other casting roll to provide a desired nip, or gap between the rolls in the casting position.

The casting rolls 12 are counter-rotated through drive shafts by an electric motor and transmission (not shown) mounted on the main machine frame. The casting rolls 12 have copper peripheral walls formed with an internal series of longitudinally extending and circumferentially spaced water cooling passages, supplied with cooling water through the roll ends from water supply ducts in the shaft portions, which are connected to water supply hoses through rotary joints (not shown). The casting rolls 12 may be between about 450 and 650 millimeters. Alternatively, the casting rolls 12 may be up to 1200 millimeters or more in diameter. The length of the casting rolls 12 may be up to about 2000 millimeters, or longer, in order to enable production of strip product of about 2000 millimeters width, or wider, as desired in order to produce strip product approximately the width of the rolls. Additionally, at least a portion of the casting surfaces may be textured with a distribution of discrete projections, for example, random discrete projections as described and claimed in U.S. Pat. No. 7,073,565 and having the tapered distribution of surface roughness described herein. The casting surface may be coated with chrome, nickel, or other coating material to protect the texture.

As shown in FIGS. 3 and 5, cleaning brushes 36 are disposed adjacent the pair of casting rolls, such that the periphery of the cleaning brushes 36 may be brought into contact with the casting surfaces 12A of the casting rolls 12 to clean oxides from the casting surfaces during casting. The cleaning brushes 36 are positioned at opposite sides of the casting area adjacent the casting rolls, between the nip 18 and the casting area where the casting rolls enter the protective atmosphere in contact with the molten metal casting pool 19. Optionally, a separate sweeper brush 37 may be provided for further cleaning the casting surfaces 12A of the casting rolls 12, for example at the beginning and end of a casting campaign as desired.

A knife seal 65 may be provided adjacent each casting roll 12 and adjoining the housing portion 53. The knife seals 65 may be positioned as desired near the casting roll and form a partial closure between the housing portion 53 and the rotating casting rolls 12. The knife seals 65 enable control of the atmosphere around the brushes, and reduce the passage of hot gases from the enclosure 27 around the casting rolls. The position of each knife seal 65 may be adjustable during casting by causing actuators such as hydraulic or pneumatic cylinders to move the knife seal toward or away from the casting rolls.

The casting rolls 12 are internally water cooled so that as the casting rolls 12 are counter-rotated, shells solidify on the casting surfaces 12A as the casting surfaces rotate into contact with and through the casting pool 19. During casting, metal shells formed on the casting surfaces of the casting rolls are brought together at the nip to deliver cast strip downwardly from the nip into the first enclosure 27. Between the casting rolls and pinch roll stand 31, the newly formed steel strip is enclosed within the first enclosure 27 defining a sealed space or atmosphere. The first enclosure 27 is formed by a number of separate wall sections which fit together at various seal connections to form a continuous enclosure wall.

The first enclosure 27 further comprises a wall section 41 disposed about the guide table 30 and connected to the frame of the pinch roll stand 31. Accordingly, the strip exits the first enclosure 27 by passing between the pair of pinch rolls 31A and passes into a intermediate enclosure denoted generally as 61 supporting an atmosphere 68.

After passing through pinch roll stand 31, the strip 21 is supported by the guide table 30 to the rolling mill 32 as shown in FIG. 5. An anti-crimping guide roll 70 may be located immediately in advance of the rolling mill 32, operable to be raised and lowered to lift the cast strip out of its straight line horizontal path so as to pass around the anti-crimping roll and to be wrapped about the upper reduction roll 32A in advance of the nip between the reduction rolls 32A.

To hold the strip down on the guide table 30 when the anti-crimping roll 70 is raised, an upper pass line roll 72 is brought downwardly to engage the strip against the guide table 30.

The intermediate enclosure 61 extends generally to the pass line roll 72. The atmosphere 68 of the intermediate enclosure 61 may be separate from the first enclosure 27, where the strip can be held in a separate atmosphere 68 in the intermediate enclosure 61. Alternatively, the atmosphere 68 in the intermediate enclosure 61 may be substantially the same as the atmosphere in the first enclosure 27.

The cast strip 21 is enclosed in a second enclosure 74 between the pass line roll 72 and the hot rolling mill 32 supporting a second enclosure atmosphere 76. The second enclosure 74 may be fitted with water spray nozzles operable to spray a fine mist of water droplets adjacent the surface of the steel strip as it passes through the second enclosure, and thereby to generate steam within the second enclosure while tending to avoid liquid water contact with the steel strip. The nozzles may be operable with a gas propellant to produce a fine mist of water. In one alternative, the gas propellant may be an inert gas such as nitrogen. The water may be supplied at around 100-500 kPa pressure, although the pressure of the water is not critical. Accordingly, the nozzles may be set up to produce a fine mist spray across the width of the strip to generate steam within the second enclosure 74.

In operation of the illustrated caster, the first enclosure 27, intermediate enclosure 61, and second enclosure 74 may initially be purged with nitrogen gas prior to commencement of casting. Prior to casting, the water sprays are activated so that as soon as the hot strip passes into the second enclosure 74 steam is generated within that enclosure so as to produce a positive pressure preventing ingress of atmospheric air. The supply of nitrogen may be terminated in the second enclosure 74 after commencement of casting.

As used in the specification and claims herein, “seal”, “sealed”, “sealing”, and “sealingly” in reference to the scrap receptacle 26, first enclosure 27, intermediate enclosure 61, second enclosure 74 and related features is not a complete seal so as to prevent leakage, but rather is usually less than a perfect seal as appropriate to allow control and support of the atmosphere within the enclosure as desired with some tolerable leakage. As such, the supply of nitrogen into the first and intermediate enclosures may be controlled to limit the amount of air ingress.

The cast strip will take up oxygen present in the first, intermediate and second enclosures forming scale on the strip. The scale on the strip surface increases the emissivity of the surface of the strip, i.e. the relative ability of the surface to emit energy by radiation. Radiant heat transfer increases with increasing emissivity, and thereby the strip temperature reduces as scale forms on the strip surface. To control the oxidation of the strip surface and maintain a desired temperature at the hot rolling mill, a reducing atmosphere may be maintained in the first enclosure 27 and the intermediate enclosure 61. Controlling oxidation of the surface of the strip controls heat transfer by radiation, and thereby the temperature drop of the strip before the hot rolling mill. Additionally, limiting scale formation reduces surface imperfections caused by rolled-in scale.

The measured temperature in the first enclosure 27 adjacent the pinch rolls 31 may be between about 1800 and 2400° F. As shown by the line identified by reference “A” in FIG. 6, at a temperature of about 2200° F., in an atmosphere containing CO and CO₂ oxidation will be substantially reduced where the CO/CO₂ ratio is greater than about 3, i.e. 75% CO in the CO+CO₂ mixture for Fe°. However, for steels deoxidized using silicon, i.e. silicon killed steels, the iron reacts with silicon oxides to form fayalite, FeSiO₂, and/or other iron silicates forming a protective layer on the steel surface. For silicon killed steels, in a CO+CO₂ atmosphere higher levels of CO₂ may be tolerated while maintaining a reducing atmosphere. As shown in FIG. 7, the protective layer of iron silicates inhibits the oxidation of the steel. The point at which FeO forms along the C+CO₂=2CO equilibrium line is identified as point “B” in FIG. 7. This point is approximately 60% CO in the CO+CO₂ mixture. As such, there is substantially reduced oxidation at a CO/CO₂ ratio of greater than about 1.5, i.e. at least 60% CO in the CO+CO₂ mixture for silicon killed steel.

The atmosphere in the first enclosure 27 may include CO₂ with the ingress of air into the first enclosure. CO is present in the first enclosure 27 as shown below in TABLE 1 from reaction of oxygen and carbon from the molten steel above the nip exiting the steel adjacent the nip. Some amount of CO may be formed from CO₂ reacting with carbon on the surface of the steel. The atmosphere in the first enclosure 27 may have a CO to CO₂ ratio of at least 1.5 during steady state operation. Alternatively, the atmosphere in the first enclosure has a CO to CO₂ ratio of at least 2.5 during steady state operation. In yet another alternative, the atmosphere in the first enclosure has a CO to CO₂ ratio of at least 3 during steady state operation. As used in the specification and claims herein, “steady state operation” does not mean strictly unchanging over time, but is instead an average of data collected during a period of normal casting operation in which no significant caster or casting parameters are changed, for example removal of the scrap box or change of casting speed.

Optionally, hydrogen may be provided in the first enclosure to provide a reducing atmosphere. The amount of hydrogen may be greater than 0.1% in the first enclosure 27.

Experimental data showing the average amounts of carbon monoxide and carbon dioxide in the enclosures are shown in TABLE 1 during casting of silicon killed steel. As shown in TABLE 1, the CO/CO₂ ratio is between about 2 and 6 in the first enclosure 27 and the intermediate enclosure 61 providing a reducing atmosphere.

TABLE 1 Carbon Carbon Oxygen Water Hydrogen Monoxide Dioxide (O₂) (H₂O) (H₂) (CO) (CO₂) (vol %) (vol %) (vol %) (vol %) (vol %) Casting Pool <0.05% <0.05% 0.05 to 0.5%  0.1 to 0.5% <0.1% First Enclosure, 0.1 to 0.25% <0.05% 0.05 to 0.25% 0.1 to 0.3% <0.05% Loc. #1 First Enclosure, 0.1 to 0.25% <0.05% 0.05 to 0.25% 0.1 to 0.3% <0.05% Loc. #2 Intermediate 0.1 to 0.5%  <0.5% 0.05 to 0.25% 0.1 to 0.3% <0.05% Enclosure Second Enclosure 2 to 8% 5 to 10% 0.05 to 0.25% <0.05% <0.05%

The oxygen content in the first enclosure 27 is less than 0.5% by volume. Alternatively, the oxygen is less than 0.25% by volume in the first enclosure 27. Air ingress into the first enclosure 27 adds oxygen and argon to the reducing atmosphere. Nitrogen may be introduced into the first enclosure 27 such that the O₂ to Ar ratio is less than 18 during steady state operation while maintaining desired oxygen levels below 0.5%. Alternatively, the reducing atmosphere in the first enclosure having an oxygen level of less than 0.5% has an O₂ to Ar ratio between about 10 and 15 during steady state operation. When excess air enters the enclosure, additional oxygen and argon enter the enclosure. However, excess oxygen in the air is consumed by the steel, lowering the O₂ to Ar ratio. Moisture in the first enclosure may be less than 0.2% water vapor by volume to further inhibit oxidation.

As discussed above, water is provided in the second enclosure 74. A desired amount of oxidation may be provided on the strip to improve the life of the reduction rolls 32A and to improve the cast strip surface. The second enclosure atmosphere 76 may include hydrogen, water, and oxygen. The total of oxygen, water vapor and hydrogen in the second enclosure atmosphere 76 may be greater than 8% by volume.

In one application, the cast strip may include the following composition, with the balance being iron and inevitable impurities (by weight):

Carbon 0.02-0.04% Manganese 0.6-0.9% Silicon 0.15-0.24% Sulfur 0.001-0.003% Phosphorus 0.01-0.018% Copper 0.26-0.37% Chromium 0.09-0.17% Nickel 0.09-0.16% Molybdenum 0.03-0.04%

Alternatively, the composition of the cast strip includes a carbon content of less than 0.5% by weight. In yet another alternative, the composition may have a carbon content of less than 0.2% by weight.

While the invention has been described with reference to certain embodiments it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiments falling within the scope of the appended claims. 

What is claimed is:
 1. A method of continuously casting steel comprising: (a) forming a casting pool of molten steel comprising a carbon content of less than 0.5% by weight on casting surfaces of a pair of internally cooled casting rolls having a nip formed between them, (b) counter rotating the casting surfaces of the casting rolls toward each other to produce a cast steel strip moving downwardly away from the nip between the casting rolls, (c) guiding the cast strip through a first enclosure adjacent the casting rolls as the strip moves away from the casting rolls, the first enclosure having a reducing atmosphere containing carbon monoxide of at least 0.1% and optionally hydrogen of at least 0.1%, (d) establishing said reducing atmosphere in the first enclosure during steady state operation to control ingress of atmospheric air so as to maintain said atmosphere in the first enclosure with a CO to CO₂ ratio of at least 1.5.
 2. The method of continuously casting steel as claimed in claim 1 where the reducing atmosphere in the first enclosure also contains argon with an O₂ to Ar ratio of less than 18 during steady state operation.
 3. The method of continuously casting steel as claimed in claim 2 where the reducing atmosphere in the first enclosure has an O₂ to Ar ratio between 10 and 15 during steady state operation.
 4. The method of continuously casting steel as claimed in claim 1 where the atmosphere in the first enclosure has a CO to CO₂ ratio of at least 2.5 during steady state operation.
 5. The method of continuously casting steel as claimed in claim 1 where the molten steel comprises a carbon content of less than 0.1% by weight.
 6. A method of continuously casting steel comprising: (a) forming a casting pool of molten steel comprising iron and silicon on casting surfaces of a pair of internally cooled casting rolls having a nip formed between them, (b) counter rotating the casting surfaces of the casting rolls toward each other to produce a cast steel strip moving downwardly away from the nip between the casting rolls such that iron silicate is formed on the casted surface of the cast strip, (c) guiding the cast strip through a first enclosure adjacent the casting rolls as the strip moves away from the casting rolls, the first enclosure having a reducing atmosphere containing carbon monoxide of at least 0.1% and optionally hydrogen of at least 0.1% to control ingress of atmospheric air so the atmosphere in the first enclosure has a CO to CO₂ ratio of at least 1.5 during steady state operation, (d) moving the cast strip through pinch rolls and thereafter through a second enclosure upstream of a roll mill where the cast strip reduction is at least 10%, the atmosphere in the second enclosure being a controlled atmosphere containing a total of oxygen, water vapor and hydrogen of greater than 8% by volume during steady state operation.
 7. The method of continuously casting steel as claimed in claim 6 comprising in addition moving the cast strip through an intermediate enclosure between the first enclosure and the second enclosure, the intermediate enclosure being a reducing atmosphere containing carbon monoxide and/or hydrogen of at least 0.1%.
 8. The method of continuously casting steel as claimed in claim 6 with a measured temperature in the first enclosure adjacent the pinch rolls between 1800 and 2400° F.
 9. The method of continuously casting steel as claimed in claim 6 where the reducing atmosphere in the first enclosure also contains argon with an O₂ to Ar ratio of less than 18 during steady state operation.
 10. The method of continuously casting steel as claimed in claim 6 where the reducing atmosphere in the first enclosure has an O₂ to Ar ratio between 10 and 15 during steady state operation.
 11. The method of continuously casting steel as claimed in claim 6 where the atmosphere in the first enclosure also contains argon with an average CO to CO₂ ratio of at least 2.5 during steady state operation.
 12. The method of continuously casting steel as claimed in claim 6 where the molten steel comprises a carbon content of less than 0.1% by weight. 